1. Field of the Invention
The present invention relates to a technique for performing a multi-layer welding by using a robot having a torch for arc welding mounted thereon, and more specifically relates to a technique for performing adaptive welding control by using a laser sensor during multi-layer welding.
2. Related Art
A technique for making a robot to track a weld line by using data which has been obtained by sensing the weld line in advance by a laser sensor mounted on the robot together with a torch for arc welding is widely known as a real time tracking control by a robot. Further, this real time tracking control is applied to a robot which performs a multi-layer welding.
In the case of making the robot on which the laser sensor is mounted to perform a multi-layer welding, the data concerning the weld line can be obtained by sensing of the laser sensor while the robot moves for welding for a first layer. In the conventional art, a control method is employed, in which a part of or all of the data obtained by sensing during the first-layer welding are stored within a robot controller and the robot is moved, by reading the previously stored data, for welding for a second and subsequent layers (for example, refer to Japanese Patent Publication, Kokai No. 6-320462). The above method is greatly useful in keeping a good quality of the multi-layer welding, since robot moving paths in a second and subsequent layers are suitably controlled.
On the other hand, the quality of the arc welding is also influenced by welding conditions such as a welding current, a welding voltage, a welding speed (a moving speed of a robot), an offset amount of a tip position of a welding torch, etc. Particularly, in a butt welding or a lap welding, a suitable value of each of these conditions is influenced by a size of a gap (hereinafter referred to as "a gap width") existing between welding joints (that is, a gap between workpieces to be welded). For example, since a suitable width of a weld bead is normally changed in accordance with the gap width, it is necessary to change the welding condition accordingly. However, a gap width is not always constant throughout a weld section. Then, there is suggested a method in which, for the welding in accordance with the real time tracking method employing a laser sensor, the gap width is also detected so that the welding conditions such as a welding current and a welding voltage can also be controlled on real time basis depending on the detected gap width (refer to Japanese Patent Publication, Kokai No. 7-80643). A method of arc welding where welding conditions are controlled in accordance with a transition of a gap condition is generally called "adaptive welding control".
In the multi-layer welding, controlling of welding conditions in accordance with a gap width in welding for a second and subsequent layers will inevitably become necessary. For example, in the case where the gap width of a welding joint is large, it is naturally necessary to increase the width of the weld bead not only for the first layer but also for the second and subsequent layers accordingly. However, in order to satisfy this requirement, it is considered necessary that an adaptive welding control should be applied to welding for a second and subsequent layers.
However, in welding for a second and subsequent layers using a conventional multi-layer welding method, the weld line detection by the laser sensor has not been used in general, so that an adaptive welding control described in the above Japanese Patent Publication, Kokai No. 7-80643, has never been applied to a welding for a second and subsequent layers in the multi-layer welding.
Thus, in order to perform an adaptive welding control in welding for a second and subsequent layers in the multi-layer welding, it is considered to be preferable that the detection by a laser sensor be performed in welding for a second and subsequent layers in the same manner as that in welding for a first layer. However, if this method is used in multi-layer welding, the bead gradually swells as welding proceeds in the second layer, the third layer, and so on, it is difficult, and may be impossible, for the laser sensor to clearly discriminate an edge of the welded joint. In this case, even if the laser sensor can detect the edge of the welded joint in the welding for a second and the following layers, an operation efficiency will deteriorate as the laser sensor has to repeat the same operation for detecting the gap width in welding for every layer in the multi-layer welding.